1) Field of the Invention
This invention relates to a control system for an automatic transmission, especially to a control system for servo hydraulic pressure adapted to have frictional engagement means engaged or released for changing the gear position of an automatic transmission.
2) Description of the Related Art
An automatic transmission mounted on a vehicle is generally provided with plural planetary gear mechanisms. Upon shifting the speed, shifting elements (gears, carriers, ring gears) of these planetary gear mechanisms are connected in accordance with engagement or release of frictional engagement means to establish a different power flow. In such transmissions, it has conventionally been conducted to provide a feed line of each frictional engagement means with an accumulator with a view toward reducing a shock caused upon engagement of the frictional engagement means, so that an increase in servo hydraulic pressure at the time of the engagement is controlled using the pressure-accumulating function of the accumulator.
In such an automatic transmission, it may be necessary, depending on the arrangement of planetary gear mechanisms, to simultaneously have frictional elements of plural frictional engagement means engaged upon change-over from a non-running range to a reverse range for backward running. Even if one attempts to achieve such simultaneous engagement by relying upon the pressure-accumulating function of accumulators as in the conventional technique, it is difficult as a matter of fact to synchronize the timings of engagement of the individual frictional elements because of variations in the piston strokes of servo means for the respective frictional engagement means. As a result, a rotational change occurs at the shifting part at every engagement of the frictional elements so that a shift shock takes place.